Method of checking the printing quality of an ink jet printer

ABSTRACT

The invention provides a method for checking the printing quality of an ink jet printer. A control frame is printed independently of a marking message. This frame is, for example, formed of 15 dots of which the charge increases from the first to the fifteenth droplet. The distance (G 1 , G 2 , G 3 ) separating, respectively, the droplets (1) and (8), the droplets (8) and (15) and the droplets (1) and (15) are permanently maintained so as to correspond to the two relationships: G 1  G 3  =k 1  and G 2  /G 2  =k 2 , where k 1  and k 2  are constants.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of checking the printing of an ink jetprinter.

2. Description of the Prior Art

The applications of this type of printer, in the industrial markingfield in particular, increase regularly. Purely by way of illustration,there may in particular be mentioned the printing of "use by" dates onagro-alimentary products or batch numbers. Such marking is carried outmore and more using ink jet printers of the "continuous deflected inkjet" type. The correct operation of such printers is governed by anumber of relatively complex parameters which must be perfectlycontrolled if it is desired to obtain irreproachable printing quality.Despite the numerous improvements introduced these latter years in suchprinters, they are not proof against drifting of one or other of theparameters governing their operation, which may result in degradation ofthe quality of the writing. It is further known that, in most cases,such degradation of the writing quality has a typology in relation withthe defective parameter. For example, the absence of an impact ofdroplets on the support to be printed may mean that the printing headneeds cleaning. Similarly, to a fault in the positioning of the dropletsthere may correspond an abnormal ejection speed of the droplets or anincorrect speed of the movement of the head. Another problem may arisein so far as the size of the droplets is concerned. Furthermore,manufacturing or packing lines are more and more automated andpermanency of the quality appears as of prime importance. Finally, sincethe marking function comes at the end of the production line, anydefective marking results in unacceptable losses.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome these problems bydescribing a method and device for anticipating the imminence of abreak-down of quality;

by identifying the break-down of quality in the shortest possible timeafter its appearance;

stopping the manufacturing line or replacing the marking machine beforeit is too late.

The invention relates more precisely to a method of checking theprinting quality of a jet printing head characterized in that itconsists:

in creating a frame for checking at different times and positions of themarking;

in reading this control frame so as to compare it with a referenceframe.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following explanationsand the three accompanying figures which schematically illustrate threevariants of construction of a device for implementing the method of theinvention, wherein:

FIG. 1 is a control frame of a matrix comprising a column of three dots;

FIG. 2 is a control frame formed of a matrix of five dots; and

FIG. 3 is a control frame of fifteen dots.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One of the features of the method of the invention resides in the factthat it consists on the one hand in creating a specific control frame,at given times and/or positions and in reading same so as to compare itwith a control frame of the same type serving as reference.

The control frame may be independent, i.e. independent of the text to beprinted or connected to the printer or other control members of theline. The pattern to be checked is defined and specified to the controlsystem in real time.

According to another feature of the invention, the control frame isformed of a series of dots which can be printed either systematically,or regularly at a fixed frequency, or regularly at a frequency resultingfrom an apprenticeship procedure related to the specific conditions ofuse. This control frame is therefore printed in addition to the usefulmessage, in general before the latter but not limitatively.

According to another feature of the invention, the control frame isdefined as a frame formed of a succession of remarkable dots. These dotsare positioned so that checking thereof makes it possible to guaranteethat the printing is correct with sufficient probability. For a givenframe and under normal operating conditions, the position of thedifferent impacts is perfectly well known and unique within a giventolerance. If the operating conditions of the head worsen (particularlyat the printing, ink, droplets charge voltage, deflection voltagelevels), this causes a modification of the positioning of the differentimpacts outside fixed tolerances. This is detected by a control meanswhich reveals incorrect operation of the printing machine.

In a variant of the method of the invention, a control frame is formed,as shown in FIG. 1, of a matrix comprising respectively a column ofthree dots (a,b,c) and, as shown in FIG. 2, five dots (d,e,f,g,h).

In the first case (FIG. 1), variations of the distance (x) separatingtwo end dots and variations in the two distances (d₁) and (d₂),separating dot (b) from dot (a) and dot (b) from dot (c) are measured.

In the second case, the variations of the height (z) of the matrix offive dots and the variations of the distances (d₃, d₄, d₅, d₆) between,respectively, the central dot (d) and dots (e,f,g,h) are measured. Anydrift of (z) as well as of ratios (d₃ /d₆) and (d₄ /d₅) is significantof an operating fault.

In a second variant of the method of the invention, the control frame isformed of at least three printed droplets whose recognition makes itpossible to guarantee, with a sufficiently high rate of probability,good printing quality. The distribution of the dots is as follows:

a droplet charged at the minimum voltage;

at least one droplet charged at an intermediate voltage;

a droplet charged at the maximum voltage.

The voltage of the droplets is sufficient to make a significantmeasurement possible.

FIG. 3 is an illustration of such a control frame comprising fifteendots, going then from 1 to 15, droplets (1) being the least deflectedand droplets (15) the most deflected.

Such printing of the control frame, as was mentioned above, is, forexample, periodic before each message. A conventional recognition deviceformed, for example, from a lighting source, a sensor (CCD) with itsoptical system and associated processing electronics is provided forreading this control frame. The sensor (CCD) is focussed on the printedsupport at the position of impact of the droplets. This arrangementmakes it possible to read the control frame whether the support ismoving or stopped. In the first case, a synchronization signal causesthe image of the control frame to be picked up at the moment when itarrives on the support.

The results can be used for checking that printing of the control frameis correct or not under the operating conditions of the machine.

In normal use, the operator may adjust the deflection voltage of thedroplets and cause the printing distance to vary, which results in bothcases in expansion of the frame heightwards. However, the relativeposition of the different droplets is substantially maintained if acertain charging order is respected: for example, if the droplets aresent in increasing charging order. By applying this property to thecontrol frame illustrated in FIG. 3, the following results are obtained:

G₁ =distance between impact of droplets 1 and 8;

G₂ =distance between impact of droplets 8 and 15;

G₃ =distance between impact of droplets 1 and 15.

Under normal operating conditions (speed of the jet and chargingcorrect), independently of the value of the deflection voltage (specificrange) and of the specified printing distance, the followingrelationships can be written, provided that they remain within aspecified range:

    G.sub.1/G3 =k.sub.1 and G.sub.2/G3 =k.sub.2.

(k₁) and (k₂) are known constants whose values are the image of acorrect deflection, so correct printing.

If we add to said control frame a second control frame, the controlmeans may measure the distance between the control frames, so theprinting speed which depends on the speed of the conveyor if the head isfixed or the speed of the head if the conveyor is fixed. It may thusdetect in addition to printing defects, an anomaly in the travellingspeed.

The invention applies, as was said above, to any industrial markinginstallation using the ink jet technique.

We claim:
 1. A method for checking print quality of an ink jet printerwhich comprises the steps of:creating a controlled frame for checking amarking operation at different times and positions wherein said controlframe is formed of a plurality of dots printed at a frequency related tooperating conditions of said printer wherein said creating stepcomprises forming said control frame comprising a matrix of three dotsincluding a middle dot, a first end dot and a second end dot; andreading said control frame so as to compare said control frame with areference frame wherein said reading comprises measuring each distanceseparating said first end dot and said end dot, said middle dot and saidfirst end dot, and said middle dot and said second end dot,respectively.
 2. A method for checking print quality of an ink jetprinter which comprises the steps of:creating a control frame forchecking a marking operation at different times and positions whereinsaid control frame is formed of a plurality of dots printed at frequencyrelated to operating conditions of said printer, wherein said creatingstep comprises forming said control frame comprising a matrix of fivedots including a central dot and four surroundings dots; and readingsaid control frame so as to compare said control frame with a referenceframe, wherein said reading step comprises measuring a height of thematrix and each distance separating said central dot and each of saidsurrounding dots, respectively, so as to detect any variation of theheight of said matrix, and any variation in ratios of said respectivedistances separating said central dot and each of said four surroundingdots.
 3. A method for checking print quality of an ink jet printer whichcomprises the steps of:creating a control frame for checking a markingoperation at different times and positions wherein said control frame isformed of a plurality of dots printed at a frequency related tooperating conditions of said printer, wherein said creating stepcomprises forming said control frame comprising at least three dropletsand wherein said at least three droplets are charged at minimum voltage,at an intermediate voltage and at a maximum voltage, respectively; andreading said control frame so as to compare said control frame with areference frame.
 4. A method for checking print quality as defined byclaim 3, wherein said control frame comprises fifteen dots of increasingcharge, and each distance between an impact of a first droplet and animpact of an eighth droplet, between said impact of said eighth dropletand an impact of a fifteenth droplet, and between said impact of saidfirst droplet and said impact of said fifteenth droplet, respectively,is maintained such that a first ratio of said distance between saidimpact of said first droplet and said impact of said eighth droplet tosaid distance between said impact of said first droplet and said impactof said fifteenth droplet is constant, and a second ratio of saiddistance between said impact of said eighth droplet and said impact ofsaid fifteenth droplet to said distance between said impact of saidfirst droplet and said impact of said fifteenth droplet is alsoconstant.
 5. A method for checking print quality of an ink jet printerwhich comprises the steps of:creating a control frame for checking amarking operation at different times and positions, wherein said controlframe comprises a series of three dots; reading said control frame tomeasure distances separating each of a first dot and a third dot, saidfirst dot and a second dot, and said second dot and a third dot,respectively; and comparing said control frame with a reference frame todetect variations in said measured distances.
 6. A method for checkingprint quality of an ink jet printer which comprises the stepsof:creating a control frame for checking a marking operation atdifferent times and positions, wherein said control frame comprises amatrix of five dots, including a central dot and four surrounding dots;reading said control frame to measure a height of said matrix and eachdistance separating said central dot and each of said four surroundingdots, respectively; and comparing said control frame with a referenceframe to detect a variation in said height of said matrix as well asvariation in ratios of said respective distances between said centraldot and each of said four surrounding dots.
 7. A method for checkingprint quality of an ink jet printer which comprises the stepsof:creating a control frame for checking a marking operation atdifferent times and positions, wherein said control frame comprises atleast three printed droplets, and said at least three printed dropletsare charged at a minimum voltage, an intermediate voltage, and a maximumvoltage, respectively; reading said control frame; and comparing saidcontrol frame to a reference frame.
 8. A method for checking printquality as defined by claim 7, wherein said creating step comprisesprinting fifteen dots of increasing charge such that each distancebetween an impact of a first droplet and an impact of an eighth droplet,between said impact of said eighth droplet and an impact of a fifteenthdroplet, and between said impact of said first droplet and said impactof said fifteenth droplet, respectively, is maintained such that a firstratio of said distance between said impact of said first droplet andsaid impact of said eighth droplet to said distance between said impactof said first droplet and said impact of said fifteenth droplet isconstant, and a second ratio of said distance between said impact ofsaid eighth droplet and said impact of said fifteenth droplet to saiddistance between said impact of said first droplet and said impact ofsaid fifteenth droplet, is also constant.